1. Field of the Invention
The invention pertains to an integrated radio detection, navigation, and communication system for cooperatively affording safe navigation at sea and more particularly relates to marine radar interrogator-transponder communication systems for use between cooperating vessels or with shore stations, permitting early detection and identification of intruding vessels such as may follow dangerous courses relative to own ship or shore stations.
2. Description of the Prior Art
The literature has dealt extensively with problems associated with safe and efficient navigation in crowded waters of modern marine vessels. For some time, it has been recognized that the annual increase in marine traffic density, in the size and speed of ships, and in their earning power per unit of time has been increasing the need for ship's operators to have complete awareness at all times of the presence of other ships, including small ones, at greater and greater distances from own ship. There has also been a growing need for positive identification of such vessels and for improved methods of navigation and piloting so that marine traffic may move during poor weather conditions and poor visibility, yet with improved safety. Improved flow of information is desired not only between ships, but also between ships and shore installations designed to control vessel traffic with the same goals of increased flow and improved safety. There has long been recognized an increasing need to achieve such goals in an inexpensive manner requiring the use of only a single integrated device on ship board.
In the U.S. Pat. No. 3,806,922 to E. J. Isbister for "Marine Radar Interrogator-Transponder Target Detection, Identification, and Range Measurement System", issued Apr. 23, 1974 and assigned to Sperry Rand Corporation, the merits and shortcomings of shipboard and shore based radar systems for providing these functions have been discussed in some detail. It was observed that radar alone suffers from a number of limitations which must be overcome if the aforementioned goals are to be achieved. One of them lies in the inability of radar to detect other ships, particularly small ones, in inclement weather just when the information is most needed. The radar picture is often cluttered with sea and rain echoes which mask wanted targets. While conventional radar instruments have been of great assistance in ship navigation and in collision avoidance, the typical radar presentation is often difficult to interpret in bad weather.
In the aforementioned Isbister patent, it was recognized that early warning of the presence of an intruding vessel, its identification, and communication with it are requisites for prevention of accidents between vessels, especially in crowded traffic and in the instance of ships having limited maneuverability. Large cargo vessels are slow to respond to applied rudder or to propeller thrust. Such vessels travel large distances before appreciable changes in course or speed can be effected. The greater the tonnage of the vessel or the greater its speed, the greater is that distance. On the other hand, at the reduced speeds often used in heavy traffic, the ship's control may become even less effective. Because of the unwieldy nature of the supership and of the natural desire of all ship masters to keep their ships moving even under conditions of poor visibility, it is necessary particularly in harbors, estuaries, and other narrow waters that traffic move in orderly patterns and follow precise routes. For this to be done safely and successfully, improved means of ship position fixing are necessary.
In the aforementioned Isbister patent, consideration was given to combining features of the conventional marine radar and radio beacon arts in such a way as to overcome their defects, particularly major disadvantages of such systems connected with serious mutual interference situations when large numbers of interrogations are made in the same general time period and eliminating saturating responses generated when a large number of unsynchronized beacon replies obscure the radar or other navigation display, degrading distinguishability of the elements of the display. In general, the more ships found with prior art radar-beacon systems in the vicinity of own ship, the worse these types of clutter became, the adverse situation again resulting just when reliable detection, identification, and communication are most needed by the ship's operator.
The aforementioned Isbister patent, with respect to which the present invention is an improvement, provided an improved radar, interrogator-transponder, navigation and marine piloting equipment of unitary nature affording improved target detection, positive target-identification, and precise range measurement for safe navigation in pilotage waters. The prior invention involved the use of cooperating interrogator-transponders which may be shipborne or may be mounted on fixed obstacles for navigation and collision avoidance purposes. The prior invention is also readily used cooperatively with radar systems of the type conventionally used for marine navigation purposes. The prior Isbister invention overcame many disadvantages inherent in pilotage when using only a conventional marine radar system and additionally is relatively free of susceptibility to mutual interference and other operational problems associated with prior art radar-beacon navigation systems. The prior invention made feasible early target detection, positive target identification, and precision target range measurement under a range of circumstances not provided by prior art radio navigation systems.